The goals of this study are to find optimal ways of characterizing neural control mechanisms that participate in the maintenance of postural stability and to develop tests that critically evaluate these mechanisms in patients with balance disorders. Recent advances in modern control theory have provided powerful methods of characterizing biological systems in terms of their input-output behavior. These methods will be used to analyze a mathematical model of human postural control system in the sagittal plane. The model represents the body dynamics as a multi-link inverted pendulum and the feedback system as a simple gain matrix relating body segment angles and angular rates to joint torques. Identification algorithms will be applied to estimate the feedback gains, and optimization algorithms will be applied to find input signals that maximize the sensitivity of response variables to variations in these gains. On the basis of this analysis, a testing procedure will be designed that, within practical constraints, provides the most sensitive measures of feedback gains. The estimated parameters of normal subjects and patients with various types of balance disorders who undergo this test will be examined to determine whether or not they can be categorized. If so, a diagnostic decision rule will be formulated that optimally discriminates amoung normal subjects and these patients.